Compositor doesn't affect Alpha plane
I think the short answer is that the image plane is indeed being taken into consideration.  I checked that by altering the colors of your image so that the butterfly is white instead of pink.  In this case, the glare node does affect the butterfly.  

I think your best bet is to really crank up the strength of the emission shader for your butterfly (I tried 200), which will give you the glare. If you want more control over the color of the butterfly image itself, you could experiment with render layers--use one layer for the butterfly image to be "glared", another layer for the color to be composited over top.